Preparation of photoconductive material



United States Patent 6 2,986,534 PREPARATION OF PHOTOCONDUCTIVE MATERIALCarolee Crawford Beutler, Milwaukee, Wis., assignor to General ElectricCompany, a corporation of New York No Drawing. Filed Aug. 22, 1957, Ser.No. 679,750

8 Claims. (Cl. 252-501) This invention relates to a photoconductivematerial and method of preparation thereof, the object being to providea material which is particularly desirable for use in a radiationamplifier.

Ordinarily, such an amplifier is referred to as a light amplifier, butthe more generic expression is used herein, since the material presentlyinvolved can be activated not only by visible radiation but by X-rays,beta rays, gamma rays, etc. The term photoconductive should beinterpreted accordingly.

In practice, photoconductive material is associated with luminescentmaterial and is subjected both to radiation of the character indicatedand to an electrical field, Whereupon the luminescent material isexcited to luminescence according to an electrical pattern establishedby radiation reaching the photoconductive material, usually through ascreen or around an object having portions opaque to the radiation. Theresult is to multiply the number of electrons emitted by thephotoconductive material and reaching the luminescent material, wherebymore photons are emitted by the luminescent material than would beemitted if such material were directly irradiated. A shadowgraph patternof radiation will, therefore, give a much brighter image than wouldotherwise be po'ssible.

Ordinarily, the photoconductive material and the luminescent materialcomprise superimposed layers and the pattern of photons emitted as theresult of irradiation of selected areas of the photoconductive materialmay either be viewed directly or used to expose a photographic emulsion.

By reason of the method hereinafter described, the photoconductivematerial herein disclosed is better adapted for this work than anypreviously known material, principally because I have discovered how toactivate cadmium sulfide with copper in a manner which produces amicrocrystalline lattice.

My method as preferably practiced is as follows:

Starting with amorphous cadmium sulfide, desirably having totalimpurities not more than a few parts per million, a small amount ofcopper activator is added. The copper activator should desirably besoluble. Copper chlo'ride, copper nitrate, copper acetate or coppersulphate are suitable separately or in mixture. The cadmium and coppersalt or salts are thoroughly mixed and desirably comprise smallparticles. Either dry mixing or mixing in a water slurry will beeifective. Desirably (but not necessarily) flux such as potassium orsodium chloride is added and mixed with the other ingredients. Drymixing may conveniently be done in a ball mill or otherwise.Alternatively, such a mill may be used to reduce the particle sizes ofmaterial which has been dried following mixing in a slurry. The sizes orparticles of component materials of the final mixture desirably do notexceed about five microns.

The mixture is then heated to a very high temperature in an atmosphereof an inert gas. Provided the particle size of the component materialsis small, the extent to which the mixture it heated will control thesize of the 2,986,534 Patented May 30, 1961 1120 C. There is no toptemperature limit except the point where the crystals become too largefor practical use.

The copper fuses into the cadmium sulfide lattice (the word fusingcommonly includes the phenomenon of migration) at the temperaturesindicated and upon cooling is tightly bound in the cadmium sulfidecrystalline lattice. After being cooled, the mixture is crushed, as in aball mill or with mortar and pestle, to reduce the materials to a finepowder without destroying the microcrystalline structure. Grindingshould be kept to a minimum. The resulting particles are washed free ofsuch activating copper salts and flux as are not bound in the lattice.This may be accomplished with Water alone although a dilute solution ofpotassium or sodium cyanide aids in the removal of the excess copper.

After the fired material has been washed and dried, it is very slightlyoxidized. This may be accomplished successfully by dusting very thinlayers of the powder onto glass sheets and heating in oxygen or air.Neither the temperature nor duration of the heating is particularlycritical, but best results have been observed within the range of 400 to600 C. The resulting powder is highly photoconductive and particularlysuitable for use in radiation amplifiers. Samples have been preparedwhich have a light-td-dark photocurrent ratio greater than 10' at 2-foot candles. Apparently each particle of the powder comprises a latticeof the cadmium and copper salts upo which cadmium oxide is coated.

A specific example of preparation is at follows: 30 cc. of a solutioncontaining 0.001 gram Cu++ as CuCl and 20 cc. of a solution containing0.1 gram NaCl/cc. are added to grams of cadmium sulfide. This isuniformly mixed and then dried. After drying, the CdS with CuCl and NaOlis further mixed and reduced to small particle size. After thoroughmixing, the powdered mixture is heated to 1120 C. for one hour in anatmosphere of nitrogen. After cooling it is crushed to a fine powder andwashed with 2% KCN and hot water, and dried. The dry washed powder isdusted onto glass sheets in thin layers and heated in air for fifteenminutes at 500 C. The resulting powder is highly photo'conductive.

The highest photoconductivity of the powder prepared as above appears tobe in the range of orange light of about 6000 Angstroms. However, manyother combinations of activator concentration, firing temperature andfiring time may be used and the resistance of the powder may be variedto suit particular applications by varying the copper concentration, thefiring temperature and the firing time.

The table below shows characteristics of typical samples ofphotoconductive cadmium sulfide composition prepared in accordance withthe method above disclosed. The table also indicates ranges which havebeen found most desirable in the compositions used and the firing Forthe purpose of the tests reportedinthe'foregoing table, the samplestested were made by suspending cadmium sulfide powder in a dielectricand placing it between two electrodes 5 mils apart and covering a smallareain the order of 0.4 square inch.

Work with cadmium sulfide crystals of large sizes has been done in thisfield, but the present procedure results in the formation of amicrocrystalline structure having great advantages. The formation of themicrocrystalline structure is believed to be attributable to the firingof small size particles of the starting material within the specifiedrange of temperature. The copper causes the photo-sensitivity and thepresence of the chloride helps to incorporate the copper in the lattice.

Photoconductive layers of this powder may be made in any desired size orshape by ordinary techniques of spraying, settling or silk-screening.The small size of the crystals and the small crystal aggregates of theresulting powder are very advantageous in this regard. The technique ofpreparation is much simpler than that required in the preparation ofphotoconductive large single crystals or vapor-deposited layers. Theresults are much more readily reproducible with accuracy than by anypreviously known method. Moreover, it is advantageous to be able to varyelectrical resistance to suit dilferent applications.

, This powder has made it possible to construct a solid state imageamplifier which, without it, Was proving difii- :cult of development.

I claim:

1. The method of preparing a photoconductive material which comprisesmixing 100 gms. of cadmium sulfide with a solution of .001.03 gm. of acopper salt and .l-.2 gm. of an alkali chloride flux, heating themixture to a temperature of 800ll50 C. and in an inert atmosphere tocrystallize the cadmium sulfide and bind copper into the crystallattices, crushing the material without destroying the cadmium sulfidecrystal structure, washing the unbound copper and its salt and the fluxfrom the material, and heating the crystals at 400-600" C. in anoxidizing atmosphere up to 15 minutes for oxidizing the crystal surfacesto a very slight degree.

2. The method of preparing a photoconductive material which comprisesmixing in the proportion of 100 gms. of cadmium sulfide with a solutionof .001.03 gm. of a copper salt and .1-.2 gm. of an alkali chlorideflux,

heating the mixture to a temperature of 800-1150" C.

for up to one hour in an inert atmosphere to crystallize the cadmiumsulfide and bind copper into the crystal lattices, crushing the materialwithout destroying the cadmium sulfide crystal structure, washing theunbound copper salt and the flux from the material, and heating 'thecrystalline material in an oxidizing atmosphere at rial whichcompri'sesmixing cadmium" sulfide vn'th a copper salt and an alkalichloride flux in the approximate proportions of parts to .00l-.03 partand .1 part respectively, reducing the mixture to small particle size,heating the mixture to a temperature in the range of 800-1150 C. in aninert atmosphere to crystallize the cadmium sulfide and bind copper intothe crystal lattices, crushing the material without destroying thecadmium sulfide crystal structure, removing the unbound copper salt andthe flux from the material, and heating the crystalline material in anoxidizing atmosphere at a temperature in the range of 400-600 C. forabout 15 minutes for oxidizing the crystal surfaces.

5. The method of claim 4 in which the mixture is reduced to no more than5 microns particle size.

6. The method 'of claim 4 inwhich the final particle size is reduced toless than 5 microns.

7. The method of preparing a photoconductive material which comprisesmixing 100 gms. of cadmium sulfide with a solution of .001.-.03 gm. of.a copper salt .and .1.2 gm. of an alkali chloride flux, drying themixture, reducing the mixture to a particle, size'of no more than 5microns, heating the mixture to a temperature in the range of 8004150 C.up to an hour in'an inert atmosphere to crystallize the cadmium sulfideand bind copper into the crystal lattices, cooling the material,crushing the material without destroying the cadmium sulfide crystalstructure, washing the material with a solution of an alkali cyanide toremove the unbound copper salt and the flux, and heating the material inair at a .tem-

perature in therange of 400-600 C. for about 15 minutes for oxidizingthe cadmium sulfidecrystal surfaces.

8. The method of preparing a photoconductive material which comprisesmixing cadmium sulfide with a solution of a copper salt inthevapproximate proportions of 100 gms. to 001-.03 gm. and with about .1gm. of an alkali chloride flux in solution, reducing the mixture toparticle size of no more than 5 microns, heating the mixture in an inertatmosphere at a temperature of approximately 1120 C. for about 1 hour tocrystallize the cadmium sulfide and bind metallic copper into thecrystal lattices, crushing the material without destroying the cadmiumsulfide crystal structure, washing the material with a dilute alkalicyanide solution to remove the unbound copper and its salt and the fluxand heatingthe material in thin layers in air for about 15 minutes at atemperature of approximately 500 C. for oxidizing the cadmium sulfidecrystal surfaces.

References Cited in the file of this patent UNITED STATES PATENTS2,651,700 Gans Sept. 8, 1953 2,727,865 Markoski Dec. 20, 1955 2,727,866Larach Dec. 20, 1955 2,755,255 Beutler July 17,1956 2,765,385 ThomsenOct. 2, 1956 2,861,903 Heimann Nov. 25, 1958 2,884,507 Czipott et a1Apr. 28, 1959

1. THE METHOD OF PREPARING A PHOTOCONDUCTIVE MATERIAL WHICH COMPRISESMIXING 100 GMS. OF CADMIUM SULFIDE WITH A SOLUTION OF .001-.03 GM. OF ACOPPER SALT AND .1-.2 GM. OF AN ALKALI CHLORIDE FLUX, HEATING THEMIXTURE TO A TEMPERATURE OF 800-1150*C. AND IN AN INERT ATMOSPHERE TOCRYSTALLIZE THE CADMIUM SULFIDE AND BIND COPPER INTO THE CRYSTALLATTICES, CRUSHING THE MATERIAL WITHOUT DESTROYING THE CADMIUM SULFIDECRYSTAL STRUCTURE, WASHING THE UNBOUND COPPER AND ITS SALT AND THE FLUXFROM THE MATERIAL, AND HEATING THE CRYSTALS AT 400-600*C. IN ANOXIDIZING ATMOSPHERE UP TO 15 MINUTES FOR OXIDIZING THE CRYSTAL SURFACESTO A VERY SLIGHT DEGREE.